1. Field
The embodiments described herein generally relate to fasteners used in aerospace applications, and more particularly relates to fasteners that provide lightning protection when used to fasten composite or other electrically conducting materials.
2. Background
Use of composites such as carbon fiber reinforced plastics is becoming more common as advancements in composite technologies increase. Use of composites allows designers to improve structural performance compared to metal structure and reduce weight. A major challenge to the use of composite structure is its susceptibility to the effects of lightning compared to metal.
Metal structure such as aluminum fuel tanks provide a layer of robust lightning protection given its high conductivity material property and low resistance between structural components when fastened together with certain metal fasteners. The high conductive properties of aluminum allow lightning currents to conduct through structure with relatively few adverse effects including rupture of aluminum skins or ignition sources within the fuel tank. Lightning protection for metal is typically achieved by ample skin thickness and fastening joints together using methods that will prevent ignitions.
Lightning protection of composite structure, such as carbon fiber reinforced plastic, is more complicated due to its higher electrical resistance and multi-layer construction. When lightning attaches to composite surfaces the lightning currents tend to be higher at the surface penetrating metal fasteners attached to underlying substructure than for metal structure. These currents may create ignition sources inside a structure like a fuel tank. (In some cases where the substructure is metal and the skins are composite a substantial amount of lightning current can flow into substructure such as a rib of a fuel tank. This could result in arcing and sparking between the fastener and the structural elements causing the projection of incendiary particles and gases into the fueled volume if not properly designed.)
These underlying components include, for example, fuel tanks, which may be metallic or of a conductive composite material, such as for example, carbon fiber reinforced plastic (“CFRP”). While metallic tanks minimize conduct currents into the aircraft substructure, lightning poses a greater potential hazard when the fuel tank is made of a conductive composite material. Generally, upper surfaces of metallic fasteners, that secure the tank and that penetrate into the fuel tank, are exposed to direct lightning attachment. As a result, there is a potential susceptibility to sparking/arcing inside the conductive composite fuel tank from these fasteners as very high lightning currents can enter the skin and substructure components of the fuel tank via the fasteners. Under certain conditions, this could result in ignition within the fuel tank.
FIGS. 1 and 2 depict the potential effects of lightning attachment 20 directly to the head of a fastener 14 used to attach a CFRP fuel tank skin 10 to metal substructure 25 of the fuel tank 15 (a portion of the tank is depicted). The conductivity of the metal substructure 25, and its multiple attachment points to aircraft structure (not shown), create favorable conditions for potentially drawing lightning currents into the fuel tank volume 30. As shown by arrows 16, energy flows along the outer surface of the skin 10 and through the skin 10. These energy flows are sufficiently high to generate “hot particles” 18 that may eject from the fasteners 14 into the fuel tank interior 30 thereby creating a hazard.
To avoid the potential for ignition sources, some fuel tanks have fastener assemblies that are capable of carrying large lightning currents without generating hot particles or sparking. Other fuel tank attachments avoid direct attachment of the fuel tank to the aircraft substructure and utilize patches over the fasteners of the tank skin, to shield the tank from lightning attachment. However, these methods present manufacturing challenges that make utilization difficult, expensive and prone to failure.
Accordingly, it is desirable to shield or otherwise protect a fuel tank with a conductive composite skin from current and voltage surges from a lightning strike. The fuel tank protection should avoid the formation of hot particles that eject into the fuel tank volume, or arcing into the tank. In addition, the protective technology should be relatively straightforward to implement in routine manufacturing processes. Furthermore, other desirable features and characteristics of the technology for lightning protection for fuel tanks will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.